SECTION 14
WEIGHT AND BALANCE
RV-8/8A
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SECTION 14: WEIGHT AND BALANCE
Weight and its distribution can severely affect performance, handling, controllability, and even structural integrity
of any aircraft. Every airplane must, for safety’s sake, be operated within its weight and balance envelope. This
section will give you basic procedures for weighing your RV-8/8A and performing weight and balance
calculations.
The forms presented and calculations performed here are only for standard operating conditions and for
Standard FAA Pilot Weights. We all know that not all pilots and passengers weight exactly 170 lbs. Performing
weight and balance calculations based on these weights provides both the pilot and the FAA with typical
anticipated loading conditions. For operating the aircraft under conditions where the average pilot and passenger
weights exceed 170 lbs., additional calculations must be performed to verify compliance with C.G. or Gross
Weight limits.
The pilot should not only be familiar with the limit load and C.G. calculations as required by the FAA, but should
also be familiar with the theory of how C.G. location affects aircraft stability and handling. Day-to-day flight
operations will not always involve loads which are exactly matched to the limit load calculations, so accurate
estimates or further calculation will be necessary to assure safe flight operations. It is possible to perform weight
and balance computations for an airplane simply by following procedures and doing the mathematics, without
really understanding all of the terms and computations. It is highly desirable that the pilot of an experimental
airplane have a good understanding of the effects that weight and balance has on the flight characteristics of the
airplane, and the reasons for C.G. limits.
Before getting into the details of performing weight and balance measurements and calculations for your RV-
8/8A, let’s review the definitions of some of the terms involved so that we can better understand the significance
of weight and balance figures.
x
Empty Weight
- Weight of the airplane including fixed ballast, unusable fuel, and oil.
x
Gross Weight
- Sum of empty weight plus crew, passengers, fuel, and baggage. It is important because of the
effect it has both on the structure and performance of the airplane. Obviously, higher gross weights will diminish
all aspects of performance, particularly take-off and climb performance. Increased weight also increases stall
speed. Higher gross weights will tend to overstress the airplane’s structure both in flight and on the ground. The
recommended gross weight of the RV-8/8A is 1800 lbs.
x
Maximum Gross Weight
- The maximum allowable operating weight, with all variable load items located such
that the Center of Gravity (C.G.) remains within prescribed limits.
x
Aerobatic Gross Weight:
The maximum weight that the structure of the airplane can support at the 6G limit of
the Aerobatic Category. For the RV-8/8A, this weight is 1600 lbs if the airplane is fitted with the “dash One” wing
supplied on Standard Kits after November 12, 2000. With the original wing, this weight is 1550 lbs.
x
Payload
- Weight of passengers and baggage.
x
Useful Load
- Weight of passengers, fuel, and baggage.
x
Center of Gravity
- The point at which the mass of an object is considered to be concentrated. (The point at
which the airframe plus all added weights are concentrated.)
x
Datum
- Arbitrary reference plane selected by the manufacturer (builder) from which all arm measurements are
made for weight and balance computations. Normally, the datum chosen will be in front of the aircraft nose so
that all arm measurements will be positive. This makes weight and balance computations easier. The weight
and balance datum for the RV-8/8A has been established at 70 inches ahead of the leading edge of the wing.
The weight and balance form and the sample calculations at the end of the chapter are based on this.
x
Arm - (or Moment Arm)
- the horizontal distance along the longitudinal axis from the datum to the C.G. of an
item being considered, or from the datum to the point where a force is applied. Normally measured in inches; aft
of datum is plus (+) and forward of datum is minus (-). In the RV-8/8A, the datum is established at a point that
makes all arms positive (+).
x
Moment
- The product of a weight or force and its moment arm (M=W x D).
x
Leveling Datum
- A point or surface on the airframe where a level can be placed to determine when the aircraft
is in a level position for weighing. For the RV-8/8A the Level Datum line is the top fuselage Longeron at the
cockpit.
x
C.G. Location (or range)
- Usually defined two different ways. One is by establishing certain positions or limits
with reference to the chord of the wing such as ‘‘between 15% and 29% of chord’’. This position is then converted
into inches by multiplying the wing chord by that percentage (for RV-8/8A, 15% =0.15, so 0.15 x 58’’= 8.7’’.
Similarly, 0.29 x 58’’ = 16.8’’, so the CG range is 8.7”-16.8” aft of the leading edge.) The second method is simply
measuring from the datum. The C.G. location with reference to datum is computed by adding these distances
RV-8/8A
SECTION 14
WEIGHT AND BALANCE
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(arms) to the distance from wing leading edge to datum. In the case of the RV-8/8A the range would be
described as 78.7 - 86.8” aft of the datum.
x
Forward C.G. Limit
- An airplane operating at or near its forward C.G. limit will have improved stall and spin
resistance and improved stall and spin recovery characteristics. It will also have higher elevator stick force and
trim force requirements, and will require more stick force for landing flare. It will have higher trim drag which will
tend to decrease speeds, but will increase the pitch stability. In general, it makes the airplane more stable and
safe, but less fun to fly.
x
Rearward C.G. Limit
- An aircraft flown at or near its aft C.G. limit will have lighter elevator stick force
requirements and will therefore be easier to rotate to a high angle of attack from which stall entry is more likely.
Stall and spin recovery at a rearward C.G. is slower and requires more corrective control action. Trim drag is
minimized and thus speeds tend to be at their highest when C.G. is more aft. In general, C.Gs in the aft half of the
range make the airplane less stable but more fun to fly, at least when maneuvering. C.Gs at or beyond the aft
limit can cause control reversals and other dangerous flight conditions.
x
Rearward ‘‘Aerobatic’’ C.G. Limits
- are often established because of the deteriorated aft C.G. stall and spin
recovery characteristics and the increased likelihood of accidental stalls and spins due to the unusual attitudes
associated with aerobatics. The aft aerobatic C.G. limit is always forward of the maximum aft C.G. limit. For the
RV-8/8A it has been established at 26.5% chord, 15.3’’ aft of the wing leading edge, or 85.3” aft of the datum.
x
Maximum Weight on Nosewheel
(Applicable to RV-8A only) - The weight on the nosewheel varies with
both gross weight and CG location and must be checked so as to be sure that it is within limits. Because of
the inter-relationship an aircraft with forward CG but low gross weight may place an unacceptably high load
on the nosewheel.
MAXIMUM BAGGAGE WEIGHT
The maximum allowable baggage is a weight determined by the structural limits of the baggage compartment
floor, and is the maximum weight which should be carried in the baggage compartment under the most ideal
conditions. For the RV-8/8A, the maximum permissible baggage limit is 50 lbs in the forward compartment and
75 lbs in the rear compartment This means that when C.G. limits and gross weight limits will permit, up to 125
lbs. of baggage can be carried. With a single occupant, additional baggage could be carried in the passenger
seat, but only if the C.G. and gross weight remained within limits, and if it were secured so it could not possibly
interfere with the pilot’s ability to operate the controls.
AIRCRAFT WEIGHING
Weigh your RV-8/8A with three platform type aircraft scales which have been checked for accuracy. At times,
good scales are not available to homebuilders and they use bathroom scales instead. Bathroom scales are often
highly inaccurate and usually do not have sufficient capacity to weigh an RV-8/8A main wheel. However, two
can be ganged together with a plank over them if no other scales are available. Because a homebuilder can
probably borrow any number of bathroom scales to use, it would be good to get 5 or 6 of them and calibrate
each, choosing the most accurate to weigh the airplane.
Regardless of the scales used, the airplane should be weighed in empty condition and in a level attitude. Level
attitude is established at the datum line which is the cockpit rails. Scales should be placed simultaneously under
both main wheels and the tail wheel. Use plumb lines or vertical levels to measure the locations of the main
wheels relative to the wing leading edge, and then convert this to an arm relative to the datum. The same applies
to the tail wheel location, which can be accurately located by dropping a plumb line to the floor and measuring
forward to the datum.
STAYING IN THE ENVELOPE
The forms at the end of this section show a sample calculation for the empty weight Center of Gravity for an RV-
8/8A. To keep all moments positive, a datum has been selected at a point forward of the most forward part of the
airplane. Only three moments must be calculated and combined to determine the C.G. position. This figure is
not in itself too meaningful, as the airplane cannot be flown in this condition, but it is important because it forms
the basis for further C.G. calculations. C.G. calculations combine the effects of all loads placed in the aircraft and
consider them as one, centered at a median point. Many different combinations of loads and locations are
possible, to achieve the same end loading result.
Increased empty weight will decrease useful load; either passenger weight, fuel weight, or baggage weight must
be reduced to remain within the permissible gross weight. This is a common practice in production light planes
where it is rare that full fuel, pilot and passengers, and baggage can all be loaded simultaneously and remain
within gross weight limits.
Several sample calculations for different loading conditions are included in this Section. The procedure is to enter
the desired loads into the calculation, and then check the resultant C.G. location to see if it falls within the design
limits. If not, then further calculations will be needed using varying loads until the resultant C.G. is within limits (a
simple computer spread sheet will enable the pilot to calculate an almost infinite variety of loadings accurately
and quickly. The examples in this Section were prepared using MS Excel). This will then become the limit load,
